Power generation is a water intensive process. In a conventional thermal power plant, fuel, such as natural gas, coal, oil, and the like, may be combusted in a boiler system. Heat released during combustion is absorbed into water-cooled walls of the boiler where the water boils and steam is formed. High temperature superheated steam passes into a steam turbine. The high temperature and pressure of the steam causes the steam turbine to rotate to drive an electric generator. The condensed steam is subsequently collected and returned through a series of pumps and heat exchangers to the boiler to repeat the cycle. Heat is extracted from the condensed steam by cooling water that is pumped to one or more cooling towers so that the waste heat can be released into the atmosphere through evaporation of the cooling water.
The degree of water reuse in the cooling towers is limited by dissolved solids in the water. That is, as the water evaporates in the cooling tower, the dissolved solids concentrate. When the concentration of dissolved solids becomes high enough, waste water, referred to as blowdown, is discharged from the cooling-tower. Consequently, feed water, also known as make up water, must be introduced into the cooling tower to replace the quantity lost to evaporation and blowdown. Make up water to the cooling tower is the largest water consumer in the power plant.
The boiler system is the final collection point for all corrosive and scale-producing contaminants generated upstream. These contaminants include minerals, organic material, atmospheric gases, and so forth. Water and steam in the boiler system are in constant contact with metal surfaces threatening the integrity of plant equipment. Corrosion can occur when metal ions transfer from a base metal to water and combine with oxygen to become hydroxides and solid metal hydroxides that can deposit on heat exchange surfaces, heaters, pumps, boiler tubes, turbines, and the like. The deposits interfere with heat transfer across the tubes which lowers the overall cycle efficiency, and can cause local tube overheating failures. Deposits can also significantly lower the efficiency of the turbines and, in turn, become corrosion sites when dissolved solids trapped in the deposit concentrate as the liquid boils away. Eventually, the concentration reaches highly corrosive levels and severe under-deposit corrosion occurs.
Like the cooling tower, when the level of dissolved solids in the boiler water becomes great enough, blowdown is discharged from the boiler system, often to the cooling towers, to reduce the contaminants that can otherwise cause severe scaling or corrosion problems. Consequently, feed water must be introduced in the boiler water to replace the quantity lost.
Organic material contaminants also pose a problem in boiler feed water. The breakdown of organic materials in boiler feed water can result in the formation of acetic and other organic acids that can corrode the boiler and associated boiler plumbing. Total Organic Carbon (TOC) is a measure of the amount of organic material suspended or dissolved in water. While acceptable levels of organic material may be one to six parts per million TOC in the cooling water, the TOC level in boiler feed water should be significantly lower. The degree of purification required for boiler feed water depends on the operating pressure of the boiler. The higher the boiler pressure, the higher the purity requirements. The American Society of Mechanical Engineers (ASME) has put together standards for the quality of boiler feed water at various drum operating pressures. Regarding organic materials, for a 300-pounds-per-square-inch (psi) boiler, the feed water should have less than 1,000 part per billion (ppb) of nonvolatile TOC. However, for a 2,000-psi boiler, the feed water should have less than 200 ppb nonvolatile TOC.
Competition for water resources among power generators and residential, commercial, industrial, and agricultural users is increasing. Indeed, water shortage is a chronic problem in some regions prone to drought and where population growth is increasing rapidly. Zero Liquid Discharge (ZLD) systems are becoming widely used in the power industry to address problems associated with limitations on water availability, increasing concern for conservation of fresh water supplies, environmental restrictions on discharges, and lengthy permitting processes. In a typical ZLD system, blowdown from the cooling towers is recycled into high purity water for reuse so that the liquid waste stream from the cooling towers is largely eliminated. In some cases, plant water consumption can be reduced from 10-90% with the addition of a ZLD system. This can minimize the potential environmental risk associated with plant waste streams and help improve unfavorable public perceptions of new facilities. Moreover, in areas of acute water shortages, ZLD design can help optimize the overall facility life cycle costs.
In some cases, the reduction of TOC levels to acceptable standards in ZLD systems has been limited. That is, the TOC level can be reduced low enough for use as cooling tower feed water, but the TOC level cannot be reduced low enough for use as boiler feed water. This problem is exacerbated in high pressure boiler systems, and/or when the feed water is lightly treated sewage effluent. In general, such effluent is low in alkalinity, high in sulfates and chlorides, and is very sensitive to swings in algae populations due to changing nutrient levels, water temperatures, dissolved gas concentrations, and so forth. The variations in algae populations in the effluent dictate the levels of organic materials, measured in TOC, that might eventually be input as feed water into the boiler system.
Consequently, continuing demand exists for an effective process and system which can reliably provide feed water of a desired purity for use as boiler feed water, as is required in a zero liquid discharge power plant.